Lifting hooks known generally as "C" hooks are used extensively in the steel industry for handling a wide range of steel products. In addition to handling coils of sheet, wire and plate, they may also be employed with slabs, billets and flat plates.
As a general rule, these "C" hooks must be balanced in such a way that the load-carrying face of the hook is substantially horizontal both when unloaded and when loaded over the full range of loads that the unit is designed to carry. In some designs, the preferred attitude is one in which the load-carrying face is at a slight angle to the horizontal.
Many systems have been used to achieve this consistent attitude, for example counterbalance weights, tension springs, compression springs, and multiple pick-up points.
Counterbalance weights are cumbersome, and may cause extra damage to material being handled due to the added weight and because they protrude past the tip of the hook. Counterbalance weights also reduce the "payload" of a crane, because of the added tare weight. Generally, counterweighted "C" hooks have limited flexibility as to coil sizes they can handle, and limited maneuverability. They are time consuming to operate and have safety limitations.
Most spring-balanced "C" hooks have linkage and tension springs located on the closed end side of the pivot point, resulting in more weight to be counterbalanced, adding additional tare weight to the "C" hook and requiring large springs. Tension springs are generally large and highly stressed and therefore create a safety hazard. Moreover, such designs are generally restricted as to the placing of coils into narrow spaces because of interference due to the linkage jutting out from the upper arm. Due to design and material limitations, such "C" hooks have not achieved acceptable levelling where the payload is variable.
One attempt to achieve this constancy of position for a crane hook is described in U.S. Pat. No. 2,514,307, issued July 4, 1950, to Wilton E. Boyd, and entitled "Crane Hook". In this patent, a cumbersome parallelogram linkage is provided which allows the pick-up point with respect to the "C" hook to shift between two positions, one position for the unloaded hook and the other position for the loaded condition. However, the arrangement shown in this patent is not one which automatically and necessarily reverts to the "unloaded" pick-up point when the load is removed from the "C" hook.
Another approach to the problem is illustrated in Belgian Pat. No. 566,095, issued Apr. 15, 1958 to Societe Ateliers Heuze, Malevez et Simon Reunis. This patent describes a "C" hook having a main suspension link extending upwardly from the upper arm of the hook, together with a complex arrangement of two additional links and a compression helical spring, the result of which is to exert a constant moment on the suspension link tending to move the upper end of the suspension link toward the back of the "C" hook, i.e. to a position in which the top of the suspension link is approximately aligned vertically with the centre of gravity of the unloaded "C" hook. In the loaded condition, the suspension link straightens up to a vertical position, in which position it is approximately aligned with the centre of gravity of the combination of the "C" hook and a typical load supported by the "C" hook. This patent is again very complicated, involving multiple links which add weight to the "C" hook and constitute additional members which may malfunction.